Auto merge of #26515 - quantheory:check_enum_recursion, r=nrc

Fixes #23302.

Note that there's an odd situation regarding the following, most likely due to some inadequacy in `const_eval`:

```rust
enum Y {
    A = 1usize,
    B,
}
```

In this case, `Y::B as usize` might be considered a constant expression in some cases, but not others.  (See #23513, for a related problem where there is only one variant, with no discriminant, and it doesn't behave nicely as a constant expression either.)

Most of the complexity in this PR is basically future-proofing, to ensure that when `Y::B as usize` is fully made to be a constant expression, it can't be used to set `Y::A`, and thus indirectly itself.

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